1. Field of the Invention
The present invention relates to a servo track writer equipped with a function of recording a servo pattern on a magnetic storage device, the servo pattern enabling high-precision positioning with high format efficiency, a perpendicular magnetic recording medium with such a high-quality servo pattern and a magnetic storage device.
2. Background Art
The growth of the Internet environment and newly provided data centers along with penetration of crowd computing have increased the amount of information generated in recent years. Magnet recording started to shift in 2005 from longitudinal magnetic recording using a longitudinal ring-shape magnetic head and a longitudinal magnetic recording medium to perpendicular magnetic recording, and the perpendicular magnetic recording has improved its performance using a main pole and a shield (auxiliary pole) type magnetic recording head and a magnetic recording medium called an Exchange Coupled Composite (ECC) medium achieving dramatically improved recordability due to small magnetic crystalline anisotropic energy Hk on the magnetic surface layer (this may be called a cap layer) side to achieve high-density recording. There is no doubt that magnetic storage devices such as magnetic disk devices (HDD) having the highest recording density and excellent bit cost play the leading role for storage in the “big-data era”. In order to support this, magnetic storage devices have to have larger capacity and so further increasing recording density is must.
As a magnetic recording technique for higher-density recording, microwave assisted magnetic recording (MAMR) has been proposed, in which high-frequency magnetic field in a microwave band is applied to a magnetic recording medium so as to excite precession movement of medium magnetization for magnetic recording on a perpendicular magnetic recording medium having large magnetic anisotropy while reducing the switching magnetic field. In recent years, practical micro-structured high-frequency Spin Torque Oscillator (STO) has been proposed by Japanese Patent No. 4677589 (which corresponds to US 2008/150643 A1), for example, configured to rapidly rotate spins by spin torque, thus generating high-frequency magnetic field. Japanese Patent No. 4255869 (which corresponds to US 2005/2132502011/0216436 A1) discloses a method of making a high-frequency magnetic field oscillator generate high-frequency magnetic field (circularly polarized magnetic field) rotating in the direction of the precession movement of the magnetic recording medium to be magnetization-reversed so as to be suitable for the polarity of the magnetic recording field, thus effectively inducing the magnetization reversal. In this way, research and development for implementation of the microwave assisted magnetic recording has become active.
In order to implement higher-density recording using an actual magnetic storage device, not only the magnetic recording technique as stated above but also high-precision positioning of a magnetic head at a predetermined data track are required. To this end, magnetic disk devices widely use a technique of recording a specific magnetic pattern (e.g., cyclic all-one pattern) called a servo burst pattern of several tens to 100 MHz beforehand as positional reference information during manufacturing, thus obtaining a head position signal from the pattern. JP 2011-129242 A (which corresponds to US 2011/149434 A1) discloses, as a servo model, a method using integrated servo fields. This servo model provides a sequence set that is constrained to provide some or all of a servo track mark (STM), position error signals (PES) and positional information such as the track-ID instead of a servo burst pattern including all-one information, and provides the position error signal in relation to the center of a data track through the amplitude of a signal read to adjacent sequences.
For a magnetic storage device provided with a plurality of magnetic heads, adaptive formatting is available, which uses a common servo track that is recorded collectively to absorb and compensate variations of the track width among these magnetic heads, thus optimizing track density or Track Per Inch (TPI) and linear recording density or Bits Per Inch (BPI) of a data track for each magnetic head during manufacturing and test process. Specifically, data is written on adjacent data tracks of a data track as a target, and then characteristics and a change thereof are evaluated by an Adjacent Track Interference (ATI) test, a squeeze test, a track width measurement test (747 curve method) and the like, thus optimize TPI. BPI is adjusted for each magnetic head so that an error rate of a plurality of magnetic heads on one magnetic storage device becomes as uniform as possible over the entire zone and the device capacity satisfies the specified value. This means that TPI changes continuously depending on the radial position and BPI is set for each zone (JP 2009-129482 A which corresponds to US 2009/128943 A1).